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This document describes a scenario where two branch routers are connected via an ISP and running Border Gateway Protocol (BGP) between them. The two branch routers (R1 and R2), though at different locations, share the same AS number. Once the routes arrive from a branch (R1 in this case) to the Service Provider (SP) network, they will be tagged with the customer AS. Once the SP passes it to the other branch router (R2), by default, the routes will be dropped if the other branch was also running BGP with the SP using the same AS number. In this scenario, the neighbor allowas-in command is issued in order to allow BGP at the other side to inject updates. This document provides a sample configuration that helps you to understand the Allowas-in feature in BGP.

Note: This feature can only be used for true eBGP peers. You cannot use this feature for two peers that are members of different confederation sub-ASs.

This is natural behavior because the BGP tries to avoid routing loops. For example, the readvertisement of all prefixes that contain duplicate Autonomous System Numbers (ASNs) is disabled by default.

Redistributed EIGRP routes (1.0.0.0, 2.0.0.0, 3.0.0.0) and the BGP internal route 22.22.22.22 from R1 are not received by R2 as they are originating from the same ASN across the Internet. Since R2 see its own AS number (121) in the AS-PATH, R2 does not take those routes.

This notification means that the BGP route received by the CE router has its own AS number in the AS path and is considered a router loop for the CE router. As a workaround, configure the CE router with the allowas-in feature as illustrated in the previous example.